Modulation is the fundamental process in any communication system. It is a process to impress a message (voice, image, data, etc.) on to a carrier wave for transmission. A band-limited range of frequencies that comprise the message (baseband) is translated to a higher range of frequencies. The band-limited message is preserved, i.e., every frequency in that message is scaled by a constant value. The three key parameters of a carrier wave are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal.
There are various shapes and forms of modulators. For example conventional Amplitude Modulation uses a number of different techniques for modulating the amplitude of the carrier in accordance with the information signal. These techniques have been described in detail in “Modern Analog and Digital Communication Systems” by B. P. Lathi. Similarly conventional Frequency/Phase Modulation uses a number of different methods described in a number of textbooks. In all these techniques, carrier (which is a high frequency sinusoidal signal) characteristics (either amplitude, frequency, phase or combination of these) are changed in accordance with the data (or information signal). Thus there has been two major components of a modulator. One is the information-carrying signal and the other is the high frequency carrier. A wave damping technique is described in this disclosure that dampens the ringing response of SAW filters further enabling specially modulated radio frequency carrier waves.
In any communication system, band pass filters are used to band limit the bandwidth of the signal. For example, they are used in transmitters to allow necessary signals to pass to the next stage and in receivers they are used to block any unwanted signals. Although they are integral part of any communication system and have numerous advantages, they have one major disadvantage. This disadvantage is the ringing effect of the band pass filter i.e. when a band pass filter is injected with a unit impulse signal; the filter rings for some time even after the signal is removed from the input. This ringing of the band pass filter is a function of bandwidth of the filter and in technical terms is referred to as the Impulse response of the filter. The narrower the bandwidth of the filter (higher Q) the more the ringing, and vice versa. The impulse response also dictates what the maximum allowable data rate is passing through such a filter. In a low data rate system, the ringing is not an issue, however, in a high data rate system, like the xMax system described below, the ringing severely degrades system throughput performance. Band Pass filters come in many shapes and forms. Most of the communication systems these days use SAW (Surface Acoustic Wave) filters. Saw filters save a lot of real estate on the printed circuit board. Saw filters are cheaper and easier to use than discrete Band Pass Filters (BPF). However, they inherit the same disadvantage as that of band pass filter, the ringing. This disclosure describes a method of damping the ringing response of band pass filters, including SAW filters, and shows an example of how this damping circuitry is used in an xMax type system.
Communication systems that have emerged in recent years include mono-pulse and Ultra-Wide Band communication systems. The problem with these systems is that all mono-pulse or Ultra-Wide Band communications systems form Power Spectrum Densities that tend to span very wide swaths of the radio spectrum. For instance the FCC has conditionally allowed limited power use of UWB from 3.2 GHz to 10 GHz. These systems must make use of very wide sections of radio spectrum because the transmit power in any narrow section of the spectrum is very low. Generally any 4 KHz section of the affected spectrum will contain no more than −42 dbm of UWB spectral power. Correlating receivers are used to “gather” such very wide spectral power and concentrate it into detectable pulses. Interfering signals are problematic. Since the communication system is receiving energy over a very wide spectrum, any interfering signal in that spectrum must be tolerated and mitigated within the receiver. Many schemes exist to mitigate the interference. Some of these include selective blocking of certain sections of spectrum so as not to hear the interferer, OFDM schemes that send redundant copies of the information in the hope that at least one copy will get through interference, and other more exotic schemes that require sophisticated DSP algorithms to perform advanced filtering. In addition, UWB systems have somewhat of a “bad reputation” because they at least have the potential to cause interference. A heated discourse has gone on for years over the potential that UWB systems can cause interference to legacy spectrum users.
Tri-State Integer Cycle Modulation (TICM) and other Integer Cycle Modulation techniques were designed by the inventors of this application to help alleviate this massive and growing problem which has now become known by its commercial designation, xMax. Its signal characteristics are such that absolute minimal sideband energy is generated during modulation but that its power spectrum density is quite wide relative to the information rate applied. Also, a narrower section of the power spectrum output can be used to represent the same information. The technique of wave damping disclosed herein is primarily applicable to these types of single cycle systems.
As described above, a signal passing through a BPF experiences ringing, thus any single cycle system, like xMax, capable of carrying a high date rate, will be severely affected by the ringing response of the BPF. Since the ringing response of the BPF is fixed and cannot be changed, additional circuitry has to be used that reduces, or damps, the ringing of the SAW filter making the system capable of sending high data rates. One such approach is described in this disclosure.